1. Field of the Invention
The present invention relates to a commutator for an electric motor, and more particularly to improvement of a commutator usable for an electric motor of the type adapted to be press fitted onto the support shaft of a rotor which is accommodated in the motor.
2. Description of the Prior Art
A conventional commutator of the above-mentioned type is generally constructd in such a manner that the cylindrical inner wall has an inner diameter smaller than the outer diameter of the support shaft of a rotor to keep a certain amount of press fitting allowance. After the support shaft is press fitted through the commutator while enlarging the inner wall in the outward direction, the commutator is fixedly mounted on the support shaft in the tightly fitted state. In general, the commutator includes a resin portion which is molded of phenol resin or the like material which has a reduced strength under the effect of tension. This causes an amount of press fitting allowance, that is, dimensional difference between the inner diameter of the commutator and the outer diameter of the support shaft to be limited within the maximum range of 20 to 30 microns. With an amount of press fitting allowance more than the above-noted range, the resin portion of the commutator tends to be broken during the press fitting operation. Accordingly, strictly controlled high machining accuracy is required for both the inner wall of the commutator and the outer surface of the support shaft. In recent years there has been a tendency to design commutators with a thin-walled structure on the basis of requirements for smaller dimensions and light weight of the commutator. However, commutators designed with the thin-walled structure are easily deformed or broken during machining operations on the inner wall. Moreover, such difficult machining operations on the inner wall of the commutator take very much time, resulting in remarkably reduced workability and increased production cost.
To obviate the problems as mentioned above, there was made a proposal for improving the conventional commutator as disclosed in Japanese Laid-Open Patent Application No. 15604, 1972. The proposed commutator is so designed that the inner wall has a substantially pentagonal cross-sectional shape and the support shaft of a rotor is press fitted through the inner wall while a very small amount of resin material enters into a plurality of hollow spaces between the inner wall of the commutator and the outer surface of the support shaft. This construction requires selection of special material for the resin portion which easily enters into the hollow space as mentioned above, or requires special machining on the support shaft. Obviously, this proposal cannot be applied to the case where fragile resin is used for the resin portion of the commutator. For these reasons it is found that the above proposal is less practical.